1. Field of the Invention
The present invention relates to a method for non-synchronized transmission.
2. Description of the Related Art
A random access procedure is used when a mobile station or a User Equipment (UE) turns on the power and need to access the wireless system. The random access procedure can be classified into two categories: non-synchronized random access, and synchronized random access.
The non-synchronized access is used when the UE is not uplink time-synchronized with the base station. This may happen when a UE turns on from a sleep mode or when a UE loses uplink timing synchronization. The non-synchronized access allows the base station (i.e., Node B) to estimate, and, if needed, adjust the UE transmission timing to within a fraction of a cyclic prefix. When a base station receives a random access sequence from a mobile station successfully, the base station sends the information on the successful sequence along with the timing advance (TA) information. The mobile station can then determine if its random access attempt has been successful by matching the sequence number that the mobile station used for asynchronous random access with the sequence number information received from the base station. If the sequence number matches, the mobile station assumes that its random access attempt has been successful, and the mobile station then uses the TA information received along with the sequence ID (i.e., SEQ#) to adjust its uplink timing. After the mobile station has acquired uplink timing synchronization, the mobile station can send uplink scheduling or resource request.
An uplink (UL or U/L) is the portion of a communications link used for the transmission of signals from a User Equipment to a base station. The uplink communication channel is often divided into a number of sub-bands, they are used by a User Equipment (UE) or User Equipments to transmit uplink data. In certain time slots, some of the sub-bands are used for uplink random access. In this case, these sub-bands are called random access channels (RACH's). The uplink data transmissions are synchronized, i.e. the transmission timings of uplink signals are controlled by the Node B (base station) in such a manner that timing offsets between different UEs are within a fraction of a cyclic prefix. In Third Generation Partnership Project Long Term Evolution (3GPP-LTE), the uplink is a Discrete Fourier Transform Orthogonal Frequency Division Multiple Access (DFT-OFDMA) system. It is also referred to as Single Carrier Frequency Division Multiple Access (SC-FDMA) system. Portion of the uplink channel could be allocated as RACH. The RACH signal is constructed from random sequences. These sequences are low bandwidth in nature, and they are often transmitted non-synchronously with the data.
Contemporarily, there may be narrow or non-existent guard band and channel filtering between the data sub-bands and the RACH sub-band(s). Therefore, interference may occur between the data sub-bands and the RACH sub-band(s).